Unexpected end of Pattern : Python Regex

Question

When I use the following python regex to perform the functionality described below, I get the error Unexpected end of Pattern.

Regex:

modified=re.sub(r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)
(CODE[0-9]{3})(?!</a>)',r'<a href="http://productcode/\g<1>">\g<1></a>',input)

Purpose of this regex:

INPUT:

CODE876
CODE223
matchjustCODE657
CODE69743
code876
testing1CODE888
example2CODE098
http://replaced/CODE665

Should match:

CODE876
CODE223
CODE657
CODE697

and replace occurrences with

http://productcode/CODE876
http://productcode/CODE223
matchjusthttp://productcode/CODE657
http://productcode/CODE69743

Should Not match:

code876
testing1CODE888
testing2CODE776
example3CODE654
example2CODE098
http://replaced/CODE665

FINAL OUTPUT

http://productcode/CODE876
http://productcode/CODE223
matchjusthttp://productcode/CODE657
http://productcode/CODE69743
code876
testing1CODE888
example2CODE098
http://replaced/CODE665

EDIT and UPDATE 1

modified=re.sub(r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(CODE[0-9]{3})(?!</a>)',r'<a href="http://productcode/\g<1>">\g<1></a>',input)

The error is no more happening. But this does not match any of the patterns as needed. Is there a problem with matching groups or the matching itself. Because when I compile this regex as such, I get no match to my input.

EDIT AND UPDATE 2

f=open("/Users/mymac/Desktop/regex.txt")
s=f.read()

s1 = re.sub(r'((?!http://|testing[0-9]|example[0-9]).*?)(CODE[0-9]{3})(?!</a>)', 
            r'\g<1><a href="http://productcode/\g<2>">\g<2></a>', s)
print s1

INPUT

CODE123 CODE765 testing1CODE123 example1CODE345 http://www.coding.com/CODE333 CODE345

CODE234

CODE333

OUTPUT

<a href="http://productcode/CODE123">CODE123</a> <a href="http://productcode/CODE765">CODE765</a> testing1<a href="http://productcode/CODE123">CODE123</a> example1<a href="http://productcode/CODE345">CODE345</a> http://www.coding.com/<a href="http://productcode/CODE333">CODE333</a> <a href="http://productcode/CODE345">CODE345</a>

<a href="http://productcode/CODE234">CODE234</a>

<a href="http://productcode/CODE333">CODE333</a>

Regex works for Raw input, but not for string input from a text file.

See Input 4 and 5 for more results http://ideone.com/3w1E3

general disclaimer about not processing HTML/XHTML/XML with regular expressions. — user177800
– user177800, Commented Jul 20, 2011 at 20:25
@thinkcool: edit your question to include code876 and CODE8765 examples. Note: your pattern does not attempt to restrict the number of digits after CODE. Also as suggested, use re.VERBOSE so that you can better see for yourself what it is doing. — John Machin
– John Machin, Commented Jul 20, 2011 at 23:17

John Machin · Accepted Answer · 2011-07-20 23:41:18Z

5

Your main problem is the (?-i) thingy which is wishful thinking as far as Python 2.7 and 3.2 are concerned. For more details, see below.

import re
# modified=re.sub(r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)
# (CODE[0-9]{3})(?!</a>)',r'<a href="http://productcode/\g<1>">\g<1></a>',input)
# observation 1: as presented, pattern has a line break in the middle, just after (?-i)
# ob 2: rather hard to read, should use re.VERBOSE
# ob 3: not obvious whether it's a complile-time or run-time problem
# ob 4: (?i) should be at the very start of the pattern (see docs)
# ob 5: what on earth is (?-i) ... not in 2.7 docs, not in 3.2 docs
pattern = r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)(CODE[0-9]{3})(?!</a>)'
#### rx = re.compile(pattern)
# above line failed with "sre_constants.error: unexpected end of pattern"
# try without the (?-i)
pattern2 = r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(CODE[0-9]{3})(?!</a>)'
rx = re.compile(pattern2)
# This works, now you need to work on observations 1 to 4,
# and rethink your CODE/code strategy

Looks like suggestions fall on deaf ears ... Here's the pattern in re.VERBOSE format:

pattern4 = r'''
    ^
    (?i)
    (
        (?:
            (?!http://)
            (?!testing[0-9])
            (?!example[0-9])
            . #### what is this for?
        )*?
    ) ##### end of capturing group 1
    (CODE[0-9]{3}) #### not in capturing group 1
    (?!</a>)
    '''

edited Jul 20, 2011 at 23:41

answered Jul 20, 2011 at 22:31

John Machin

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3 Comments

John Machin Over a year ago

@thinkcool: This answer answers correctly the question that you asked. Is it not worth at least an upvote?

c_prog_90 Over a year ago

I use the regex posted by you along with 'code' prog = re.compile(pattern4,re.VERBOSE) result = prog.match(mytext) print result 'code' Am I doing the right way. I get no match for my input

John Machin Over a year ago

@thinkcool: the regex pattern4 posted by me is functionally equivalent to yours, together with comments hinting why it doesn't work. You are invited to try to work out for yourself how to fix it.

Alan Moore · Accepted Answer · 2011-07-21 14:47:11Z

2

Okay, it looks like the problem is the (?-i), which is surprising. The purpose of the inline-modifier syntax is to let you apply modifiers to selected portions of the regex. At least, that's how they work in most flavors. In Python it seems they always modify the whole regex, same as the external flags (re.I, re.M, etc.). The alternative (?i:xyz) syntax doesn't work either.

On a side note, I don't see any reason to use three separate lookaheads, as you did here:

(?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?

Just OR them together:

(?:(?!http://|testing[0-9]|example[0-9]).)*?

EDIT: We seem to have moved from the question of why the regex throws exceptions, to the question of why it doesn't work. I'm not sure I understand your requirements, but the regex and replacement string below return the results you want.

s1 = re.sub(r'^((?!http://|testing[0-9]|example[0-9]).*?)(CODE[0-9]{3})(?!</a>)', 
            r'\g<1><a href="http://productcode/\g<2>">\g<2></a>', s)

see it in action one ideone.com

Is that what you're after?

EDIT: We now know that the replacements are being done within a larger text, not on standalone strings. That's makes the problem much more difficult, but we also know the full URLs (the ones that start with http://) only occur in already-existing anchor elements. That means we can split the regex into two alternatives: one to match complete <a>...</a> elements, and one to match our the target strings.

(?s)(?:(<a\s+[^>]*>.*?</a>)|\b((?:(?!testing[0-9]|example[0-9])\w)*?)(CODE[0-9]{3}))

The trick is to use a function instead of a static string for the replacement. Whenever the regex matches an anchor element, the function will find it in group(1) and return it unchanged. Otherwise, it uses group(2) and group(3) to build a new one.

here's another demo (I know that's horrible code, but I'm too tired right now to learn a more pythonic way.)

edited Jul 21, 2011 at 14:47

answered Jul 20, 2011 at 20:48

Alan Moore

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10 Comments

c_prog_90 Over a year ago

When I tried this regex, the regex matched all strings containing CODE[0-9]{3} and replaced them with http: //productcode/CODE[0-9]{3}. Is there a problem with my matching group

Alan Moore Over a year ago

I updated my answer to include a full solution. Let me know what you think.

c_prog_90 Over a year ago

Thanks a lot..This works for me. But, Since I am parsing the contents of a text file, I read the contents of that text file to string and use this regex. It replaces all the occurrences of CODE[0-9]{3} with the productcode/CODE[0-9]{3}. It does not take care of the special cases.

John Machin Over a year ago

This doesn't handle the OP's CODE69743 case.

John Machin Over a year ago

@thinkcool: You say it works for you, but it does not take care of the "special cases"? What special cases?? It works for all your test cases EXCEPT where CODE is followed by more than 3 digits.

|

stema · Accepted Answer · 2011-07-20 20:45:33Z

0

The only problem I see is that you replace using the wrong capturing group.

modified=re.sub(r'^(?i)((?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)(CODE[0-9]{3})(?!</a>)',r'<a href="http://productcode/\g<1>">\g<1></a>',input)  
                       ^                                                        ^                                                        ^
                    first capturing group                                  second one                                         using the first group

Here I made the first one also a non capturing group

^(?i)(?:(?:(?!http://)(?!testing[0-9])(?!example[0-9]).)*?)(?-i)(CODE[0-9]{3})(?!</a>)

See it here on Regexr

answered Jul 20, 2011 at 20:45

stema

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2 Comments

c_prog_90 Over a year ago

I get the same error when I use your modified regex.Is python uses a different Regex engine than that of RegExr

c_prog_90 Over a year ago

I removed (?-i) as told by John and I no more get the error, but I am not able to match with this regex.

Karl Knechtel · Accepted Answer · 2011-07-20 21:45:46Z

0

For complex regexes, use the re.X flag to document what you're doing and to make sure the brackets match up correctly (i.e. by using indentation to indicate the current level of nesting).

answered Jul 20, 2011 at 21:45

Karl Knechtel

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